To the Editor: Dr Shishehbor and colleagues1 showed that much of the mortality risk associated with SES may be explained by functional capacity and heart rate recovery. To quantify the contribution of these variables to the observed association between SES and mortality using hazard ratios (HR), they used the following formula:

Proportion of excess risk explained = [HRU−HRA]/[HRU−1], where HRU is the hazard ratio for mortality conferred by low SES but unadjusted for functional capacity and heart rate recovery, and HRA is the adjusted hazard ratio after accounting for functional capacity and heart rate recovery. This formula was taken from an article examining the contribution of surgeon volume to the relationship between hospital volume and surgical mortality.2